1. Field of Invention
The present invention relates to modulation in radio frequency transmitters and more particularly to efficient provision of mixing frequency signals.
2. Background in the Art
While the present invention has a wide range of utility, it finds particular application in wireless applications, particularly those embodied in silicon chips. A particularly important application for solid state transmitters is in mobile telephones. It is desirable to provide architecture for generating second generation or third generation mobile telephone operation. Second generation standards include CDMA (Code Division Multiple Access), TDMA (Time Division Multiple Access, IS-136), and GSM standard (Global Systems for Mobile). Third generation standards include WCDMA (Wide-band Code Division Multiple Access) and CDMA 2000.
A radio frequency transmitter must translate an analog input into a radio frequency signal and also provide for variable gain to provide for a selected input level to a power amplifier prior to transmission. In the modulation process, an analog signal is mixed with a mixing frequency signal to provide a higher frequency output signal. The mixing signal is commonly provided from a frequency synthesizer including a voltage controlled oscillator (VCO). Wireless transmitters in today's marketplace must be implemented cost-effectively in order to be competitive.
Current ways of addressing this need have particular drawbacks. In the well-known simple direct modulation transmitter, translation of the analog signal to a radio frequency signal is done in one stage, with one mixing step. CDMA and WCDMA systems require a dynamic range of 90 dB. Almost the whole range must be performed in amplifiers or attenuators working at the same frequency, nominally between 1 and 2 GHz. Consequently, the range of variable gain required of the transmitter must be accomplished in this frequency domain, which is quite difficult.
In the direct modulation transmitter, isolation between the power amplifier and VCO is minimized, and “cross-talk” between them may result, causing signal distortion. Additionally, carrier feedthrough may result in VCO signal leakage into the radio frequency band, also distorting the transmitted signal. Since the transmission frequency provided by the VCO is working on the same or on a harmonic of the transmitter output frequency, frequency pulling or inject locking of the VCO may result. There is also a risk for oscillation due to the high gain in direct modulation on the same frequency and feedback within an integrated circuit (IC) in which the direct modulator is embodied and outside the IC including a power amplifier between the modulator and a transmitting antenna and other coupling components.
The well-known double conversion transmitter addresses these problems found in the direct modulation transmitter. The initial modulation and mixing described above is done in a first stage providing an intermediate frequency output. A portion of the variable gain range is then implemented at the intermediate frequency. The intermediate frequency signal is the mixed with a second mixing frequency signal from a second frequency synthesizer and second VCO. Also, an extra intermediate frequency filter is required in the transmitter circuit to avoid production of spurious signals. The added circuits elements add significant expense to a transmitter embodied in a silicon chip. The degree of added expense could be sufficient to render such a transmitter uncompetitive in the marketplace.